This invention relates to an improved process for forming a thermoplastic foam body from an alkenyl aromatic polymeric material. Specifically, it relates to a process for forming a thermoplastic foam using finely-divided, water-containing organic vegetable matter as a blowing agent.
Thermoplastic foams are widely used in the area of thermal insulation. Of particular interest in this area are the styrene polymer foams. Thermoplastic foam material suitable for use in the thermal insulation field are desirably extruded foams having a generally uniform cell size.
Methods for producing extruded thermoplastic foamed material are well-known in the prior art. Typically, the steps of such methods comprise: (1) heat plastifying the alkenyl aromatic polymeric material to be foamed; (2) adding to the heat plastified polymeric material, while said polymeric material is under a pressure sufficient to prevent foaming, a volatile fluid foaming agent; and (3) extruding the foaming agent/polymeric material mixture into a zone of lower pressure to thereby cause the formation of a foam through volatilization of the foaming agent.
Exemplary of such methods are U.S. Pat. Nos. 2,409,910; 2,525,250; 2,699,751; 2,848,428; 2,928,130; 3,121,130; 3,121,911; 3,770,688; 3,815,674; 3,960,792; 3,966,381; 4,085,073; and 4,146,563; the teachings of which are herein incorporated by reference thereto.
One particularly desirable method for producing a stable styrene polymer foam is set forth in U.S. Pat. No. 3,960,792 to M. Nakamura. Nakamura describes a method of producing a styrenic polymer foam using a blowing agent mixture of methyl chloride and a chlorofluorocarbon.
While methods employing methyl chloride as a blowing agent produce foams having physical characteristics which are highly desirable for thermal insulation, the use of methyl chloride is undesirable in several aspects. Due to the stringent industrial hygiene requirements associated with methyl chloride, foams in which methyl chloride has been used as a blowing agent must be aged for a period of time sufficient to permit the methyl chloride to leave the cells and air to enter by an appropriate diffusion process through the cell walls. Obviously, the length of time the foamed material must be aged is dependent on the thickness of the foamed material. In those situations in which a relatively thick sheet of foamed material is produced, the necessity of a relatively lengthy aging period can present an inconvenient warehousing problem.
Due to the above-described disadvantages inherent in the use of methyl chloride as a blowing agent, much work has been done in an effort to find a blowing agent or blowing agents which will produce foams, having desirable physical characteristics but which will not have the above-described problems.
In U.S. Pat. No. 2,911,382, water is employed as a blowing agent wherein the water is provided in the form of a hydrated salt which decomposes, at an elevated temperature, to liberate water; U.S. Pat. No. 2,386,995 teaches expanding vinyl esters and polystyrene with plasticizing and any suitable blowing agent including water; Canadian Pat. No. 844,652 teaches the preparation of a foam forming styrene-acrylic acid copolymer wherein water is utilized as a blowing agent; U.S. Pat. No. 4,278,767 teaches a fire retardant styrene copolymer foam prepared by the direct injection of water as a blowing agent.
Finally, U.S. Pat. No. 4,455,272 teaches the formation of a polystyrene foam employing a blowing agent comprising a mixture of a volatile fluid blowing agent and water. Suitable volatile fluid blowing agents include propane, normal- and isobutane, the pentanes, and hexanes, and halogenated hydrocarbons such as trichlorofluoromethane, and dichlorodifluoromethane. The foams of U.S. Pat. No. 4,455,272 have desirable physical properties and avoid the problems associated with the use of methyl chloride as a blowing agent.
Nonetheless, the process taught by U.S. Pat. No. 4,455,272, and other such patents employing water as a blowing agent, have proven unsatisfactory in a number of aspects. The amount of water suitable for use as a blowing agent is limited to approximately that amount which is soluble in the polymeric material from which the foam will be formed. In the case of producing a polystyrene foam, the amount of water which can be dissolved in the polystyrene is small, more importantly, it is less than the desired optimal amount. Inclusion of an amount of water greater than that amount which will dissolve in the polystyrene leads to phase separation of the water and polystyrene. This phase separation results in a polystyrene foam product having unacceptable physical appearance and characteristics.
Until now, no method was known for incorporating into the polystyrene an amount of water greater than that amount which is soluble therein. It is desirable to find a way to employ more water than that amount which is soluble in the polystyrene and still produce a foam with acceptable appearance and physical characteristics.
Moreover, the prior methods have proven unsatisfactory in producing highly filled foam material. This is because of the difficulty encountered in extruding such highly filled materials. It is desirable to find a process suitable for producing highly filled thermoplastic foam material.
Additionally, when the foam material is to be used as thermal insulation it is desirable to produce a foam possessing good structural strength and high thermal insulation value.
It is to these goals that the present invention is directed.